Magnet assembly for magnetic developing brush and developing apparatus for electrostatic process



P 24, 1963 MOTOKI KOJIMA ETAL 3,402,698

TIC DEVELOPING BR MAGNET ASSEMBLY FOR MAGNE USH AND DEVELOPING ROCAPPARATUS FOR ELEC STATIC P Filed May 1967 United States Patent M3,402,698 MAGNET ASSEMBLY FOR MAGNETIC DEVELOP- ING BRUSH AND DEVELOPINGAPPARATUS FOR ELECTROSTATIC PROCESS Motoki Kojima, Haruo Fujii, andNohoru Kutsuwada, Tokyo, Japan, assignors to Konishiroku Photo IndustryCo., Ltd., Tokyo, Japan Fiied May 26, 1967, Ser. No. 641,516 Claimspriority, application Japan, June 6, 1966, 41/ 36,215 6 Claims. (Cl.118637) ABSTRACT OF THE DISCLOSURE A magnet assembly for forming amagnetic developing brush which can be supplied with and carry developerpowders for use in electrophotographic or electrostatic processes,comprises a main magnet and a pair of submagnets coupled to both sidesof the main magnet and arranged so that their direction of magnetizationlies substantially at right angles to that of the main magnet; each ofthe submagnets is disposed so that its inner pole has the same polarityas the active pole of the main magnet. This magnet assembly creates anintensified, uniform magnetic field of rectangular cross-section infront of and commensurate in width with the active or brush-formin pole.

The invention relates to a magnet assembly for forming a magneticdeveloping brush used in an electrophotcgraphic process or electrostaticprinting process.

A magnetic brush is conventionally employed in such processes to developan electrostatic latent image formed on a layer of light-sensitive orphotoconductive insulating material as by charging and subsequentimagewise exposure to radiation of the layer in a known manner.Specifically, a mixture of toner and carrier, e.g. iron particles, isretained in the form of a brush underthe influence of magnetic force andis brought into sliding contact with a light-sensitive layer whichcarries the latent image. The toner is transferred onto those parts ofthe layer of which the charge has not been neutralized during theexposure step.

Although the brush comprising the toner and iron particles shoulddesirably be formed with an elongated, upright tuft so that it may rubthe surface of the lightsensitive layer softly and uniformly with asubstantial area of contact to produce a visible image of high quality,it has been difficult to maintain a magnetic brush in such condition.

Thus the tuft of the brush is apt to become cut short or uneven, or itmay lie down to agglomerate, thereby causing insufiicient or excessivepressure of contact with the light-sensitive layer. All these conditionsof the tuft fail to achieve uniform reproduction of image density in theresulting image, and cause spots in the background, deposition of ironpowders to the layer or other deficiencies of development. In order toovercome these disadvantages, it has been proposed to provide anothermagnet pole on the opposite side of the light-sensitive layer forforming the toner-iron particle-brush vertically in depending form sothat its tuft may be elongated perpendicularly to the layer. However,the former proposal involves difliculty in construction when it isapplied, for example, in a copying machine using a drum, while in thearrangement of the latter, it is likely that fall of iron particles ontothe light-sensitive surface causes undesirable spots on the developedsurface.

It is an object of the invention to provide an improved magnet assemblywhich enables one to produce and main- 3,402,698 Patented Sept. 24, 1968tain a toner-iron particle-brush having an elongated, upright tuft andhaving a sufficient rubbing area, thereby allowing to obtain an image ofhigh quality.

Another object of the invention is to provide a developing unit whichavoids the above disadvantages.

According to the invention, the magnet assembly creates an intensified,uniform magnetic field in front of an active pole or brush-forming poleand the field distribution has a substantially rectangular cross-sectioncommensurate in width with the active pole. The resulting brush hastherefore a rectangular cross-section, thereby achieving a sufficientrubbing area with the surface of the light-sensitive layer that ispassed over the active pole. Thus sufficient and uniform tonerdeposition is assured. The magnetic field created on the active pole isintensified so as to prevent deposition of iron powders on to the layerand at the same time to produce and maintain an elongated brush. Theinvention thus overcomes the above disadvantages of the prior art with asimple and inexpensive arrangement.

Above and other objects, features and advantages of the invention willbecome apparent from the following description of embodiments thereofwith reference to the drawing, wherein:

FIG. 1 is a vertical section of a magnet assembly incorporating theprinciple of the invention, illustrating dis tribution of magnetic fluxdensity in a plane perpendicular to the length of a main magnet;

FIG. 2 is a perspective view of the magnet assembly shown in FIG. 1;

FIG. 3 schematically shows a developer unit, in vertical section, whichuses the magnet assembly shown in FIGS. 1 and 2;

FIG. 4 is a vertical section similar to FIG. 3 of a developer unit whichincorporates a magnet assembly in accordance with another embodiment ofthe invention; and

FIG. 5 is a vertical section of a developer unit which uses a magnetassembly in accordance with further embodiment of the invention.

Referring to the drawing, and particularly to FIGS. 1 and 2, a magnetassembly according to the invention is generally shown at 1 andcomprises a single piece of magnet A and a pair of submagnets B coupledto both sides of the magnet A. The magnet A may be similar in itsmaterial and size to that heretofore used to form a magnetic developerbrush. The arrangement of the submagnets B is such that their directionof magnetization lies substantially at right angles to that of the mainmagnet A, as is best seen in FIG. 1. In addition, each of the submagnetsB is disposed so that its inner pole has same polarity as that pole ofthe main magnet A which serves to form and retain a toner-ironparticle-brush thereon. Thus in the example shown in FIGS. 1 and 2, thesubmagnets B have their N-poles disposed inside abutting against thesides of the main magnet A which has its N-pole as the brush-formingpole. For convenience of illustration, these submagnets are indicated inFIGS. 2 to 5 only by notation of their outer poles, it being henceunderstood that such outer poles of the submagnets have oppositepolarity to that of the brush-forming pole of the main magnet A. Thepair of submagnets B may be replaced by a single annular magnet whichfits around the sides of the main magnet A.

In FIG. 1, there is shown a comparison of flux density distributionsobtained with the magnet assembly 1 of the invention (in broken lines)and with a single piece magnet A as in the prior art (in chain line).The curves were obtained by plotting points, at which the flux densitywas 400 gauss, in plane perpendicular to the length of the main magnetA. In one example in which the distribution shown was obtained, magnetswere made from anisotropic ferrite. The strength of magnetization was680 gauss at the center and 580 gauss at the edges of one pole piece ofthe magnet A when a probe was applied to the pole face. When a pair ofsubmagnets having strength of magnetization of 300 gauss were applied tothe lateral sides of this main magnet, a uniform flux density of 800gauss was obtained .over the whole region of the N-pole of the mainmagnet. When each of such submagnets has strength of magnetization of680 gauss originally, the resulting flux density immediately in front ofthe N-pole of the main magnet was nearly 1000 gauss. It will be notedthat with the magnet assembly according to the invention, the fluxdensity in front of N-pole (brush-forming pole) is much more increasedand more uniformly flattened over the entire region of the pole facethan the flux density in the prior art. On the other hand, the fluxdensity in front of S-pole and in the fringing areas of the pole isconsiderably attenuated. Numerically, the flux density decreased to 500gauss at the center and to 380 gauss at the edge of S-pole. This impliesthat while the flux density distribution is symmetrical with respect toeach pole of the single main magnet, this distribution can be biased toone side of the magnet and modified in shape by combining the pair ofsubmagnets with the main magnet in accordance with the teaching of theinvention. It is believed that the action of the combined submagnets isto press inward the laterally flaring portions of the fiux emanatingfrom the N-pole of the main magnet A. The modified distribution (brokenlines) has an increased flux density in front of the active orbrush-forming pole (N-pole) and the flux density is of uniform magnitudein front of the N-pole so that a brush having an elongated, upright tuftcan be readily produced and maintained. It will be also appreciated thatthe resulting brush will have a substantial cross-sectional area,thereby ensuring a uniform and sufficient image density. Control oversoftness of touch of the brush with the light-sensitive layer can bemade by variation of the magnitude of the flux density, which in turnmay be achieved by displacing the submagnets B vertically, as viewed inFIG. 1, along the lateral sides of the main magnet A. The length of thebrush formed varies with operation factors, but in one example in whichthe brush is formed to stand upright, the length was 7 mm.

FIG. 3 shows a developer unit using the magnet assembly in accordancewith the invention. The developer unit comprises a cylinder 2 ofnon-magnetic material and below the cylinder there is provided acontainer 3 which is also made from non-magnetic material and whichcontains the toner-iron particle mixture. As is well known, the tonerparticles are selected from materials which are easily charged bytriboelectricity by agitation. The cylinder 2 is adapted to be rotatablysupported by the end plates (not shown) of the container. In the spacewithin the cylinder 2, an arbor 4 of non-magnetic material carries aseries of magnets 5, 6, 7, 1, and 8. The magnets are fixedly arrangedradially of the arbor and are homopolar at their outer ends adjacent tothe cylinder 2. The magnet 1 is constructed similarly as mentioned inconnection with FIGS. 1 and 2 and its N-pole faces a light-sensitivesheet 10 as the latter passes over the magnet 1 in close proximity tothe cylinder. A scraper 11 is arranged in contact with the cylinderbetween the angular positions of the magnets 8 and 5.

In operation, the cylinder 2 is rotated in the direction indicated by anarrow by any suitable drive. The mixture of toner and iron particles 9is supplied into the container 3 in an amount such that the mixturepowders cover the bottom part of the cylinder 2. Thus by force ofattaction between the outer pole of the magnet 5 and magnetized ironparticles and also by electrostatic force of attraction between tonerand iron particles, the developing powders adjacent to this magnet isheld on the cylinder and carried thereon over successive magnets 6 and 7to reach the top position where they are subject to the influence of thecomposite magnet 1, thereby forming a magnetic brush 12 which, owing tothe effect of the above mentioned flux density distribution, has afavorable tuft shape. The brush thus formed faithfully develops theelectrostatic latent image on the sheet 10, It will be appreciated thatbecause the developing powders are of particle size, e.g. 10 to 20microns for iron particles and several microns for toner, they are freeto move on the cylinder in conformity to the flux distribution. Afterdevelopment, the powders on the cylinder 2 are carried over the magnet 8and then released from action of the magnetic field to be subsequentlyfreed from the cylinder by the scraper 11 to return to the container 3.The above cycle repeats until development of the sheet 10 is completed.

FIG. 4 shows another developer unit which is similar in construction tothat shown in FIG. 3 except that the main magnet A of the magnetassembly 1 is elongated to extend through the arbor 4 diametrically,thereby achieving an intensified brush-forming pole. The operation ofthe unit is similar as described in connection with FIG. 3.

In FIG. 5, there is shown further example of the developer unit whichincorporates the magnet assembly of the invention. In the example shown,there is not provided a cylinder 2 shown in FIGS. 3 and 4, but thecylindrical arbor 4 itself is adapted to rotate. The arbor can be drivenin any suitable manner as by connecting its center shaft (not shown) toan electric motor. A11 magnets provided in the arbor 4 are of sameconstruction as the magnet assembly 1 shown in FIGS. 1 and 2 and theyare embedded in the arbor along the periphery thereof at apart. Thisachieves an improved brush, of which tuft can be maintained in anoperative condition that excellently distinguishes itself over the priorart. Though in FIG. 5 each magnet assembly 1 has a separate brushassociated therewith so that during rotation of the arbor 4, the brushescome in contact with the light-sensitive sheet in turn onlyintermittently, the latent image on the sheet can be developedcontinuously, that is, without leaving any undeveloped portion. To thisend, the sheet 10 may be moved at a relatively low speed and the arborrotated at a higher speed so that the brushes repeatedly scan the sheet.Alternatively, the number or arrangement of the magnet assembliesprovided in the arbor maybe modified so that tuft portions of adjacentbrushes continuously cover the sheet. The softness with which the brushtuft formed on the magnet assembly touches the sheet can be controlledby suitable choice of magnetization intensity of the magnets used in themagnet assembly, or by displacing the small magnets B along the lateralsides of the main magnet A.

What is claimed is:

1. A magnet assembly for a magnetic developing brush comprising a mainmagnet having magnetization in a first direction and also having anactive pole, and at least one magnet attached to lateral sides of themain magnet and having magnetization in a second direction substantiallyperpendicular to said first direction, the pole of the said magnetadjacent to said main magnet being of same polarity as the active poleof the main magnet, whereby said active pole is adapted to produce auniform magnetic field in front of it.

2. A magnet assembly for forming a magnetic developing brush, comprisinga pair of first magnets having magnetization in a substantially commondirection and arranged at a distance with their poles of selected likepolarity opposing each other, and a second magnet interposed betweensaid pair of magnets in close proximity thereto and having magnetizationin a direction substantially at right angles to the first saiddirection, whereby that pole of the second magnet which has the samepolarity as that of the opposing poles is adaped to form a magneticdeveloper brush thereon.

3. A developer unit including the magnet assembly according to claim 1,wherein said magnet assembly is stationarily arranged within acylindrical rotating body of non-magnetic material with said active polefacing outward, said cylinder being adapted to be supplied with andcarry developer powders, whereby said magnet assembly acts to form amagnetic brush of developer powders on the body.

4. A developer unit according to claim 3, wherein a plurality of magnetsis arranged adjacent to the inner periphery of the cylindrical body saidmagnets having poles which cooperate with the cylindrical body to carrydeveloper powders on the body.

5. A developer unit including a plurality of the magnet assembliesaccording to the claim 1, wherein the assemblies are fixedly arranged ona cylindrical rotating body with their active poles facing outward, saidassemblies being arranged spaced apart along the periphery of thecylindrical body, said body being adapted to be supplied with developerpowders to thereby form a magnetic brush of developer powders on theactive pole of the respective magnet assembly.

6. A developer unit including at least one magnet assembly according toclaim 1, and including a developing brush which comprises toner andmagnetic carrier, said brush being formed and maintained under themagnetic influence of the active pole of the magnet assembly, said brushbeing laid under relative movement to a surface having an electrostaticlatent image.

References Cited PETER FELDMAN, Primary Examiner.

